In exhaust gas purification systems for diesel engines provided with a selective reduction catalyst (SCR catalyst), an exhaust gas device provided with a selective reduction catalyst and an oxidation catalyst (DOC) in this order from the upstream side is disposed in an exhaust gas passage. By supplying an ammonia-based solution W that forms ammonia, such as urea water, from an ammonia-based solution injection device disposed upstream the exhaust gas device to the selective reduction catalyst, NOx contained in the exhaust gas and the ammonia is selectively reacted to purify the NOx.
In these exhaust gas purification systems provided with a selective reduction catalyst, harmless urea water is supplied in an exhaust gas passage and is thermally decomposed on the catalyst to form ammonia. With the selective reduction catalyst, the ammonia selectively reduces NOx contained in the exhaust gas.
In these urea-supplying NOx purification systems, however, urea is supplied as the source of forming ammonia. Accordingly, when these NOx purification systems are widely spread, since it is necessary to supply a large amount of urea, there is a problem from the standpoint of infrastructure under the present conditions.
Consequently, for example, as described in Japanese patent application Kokai publication Nos. 2004-211679 and 2001-140630, there are proposed a NOx post-treatment device and a method for an internal-combustion engine, wherein a NOx adsorbent (lean Nox trap: LNT) or an ammonia forming catalyst is disposed upstream a selective reduction catalyst, ammonia, which is formed when the exhaust gas is made be in a high temperature rich state for regenerating the NOx adsorbent and the like, is adsorbed by the selective reduction catalyst, and the adsorbed ammonia is used to reduce NOx contained in the exhaust gas during normal operation after the end of the high temperature rich state.
This ammonia is formed by the reaction between NOx contained in the exhaust gas and NOx stored by an NOx occlusion reduction type catalyst, which is one of lean NOx catalysts, and the like, and hydrogen (H) thermally decomposed from water (H2O) or hydrocarbons (HC) in the exhaust gas under reduction conditions of low oxygen (O2).
In such NOx purification systems, however, there is such problem that, when ammonia is not stably supplied without the check of generation conditions of ammonia or quantitative control, excess ammonia is formed to be released in air, or an inadequate amount of ammonia is formed to release NOx in air. Therefore, there is such problem that stable formation of ammonia is necessary.